A cloned segment of the African green monkey (Cercopithecus aethiops) genome that contains DNA sequences homologous to the control region of simian virus 40 is being studied. This sequence, 450 base pairs in length, is embedded in a genomic DNA region that is especially rich in interspersed repeated sequences. The segment homologous to SV40 is flanked by two members of the Alu family, one of which represents a novel Alu subfamily whose members contain a potential Z-DNA forming segment. The SV40-like region, which is hypersensitive to DNase I in monkey chromatin, serves as a transcriptional start site for cellular RNA synthesis. Also, the sequence provides information for initiation of transcription from vectors constructed by molecular cloning as measured by expression of an E. coli gene after transfection of the vector into mammalian cells. Expression was measured both by the percent of cells transformed by the E. coli gene and by analysis of messenger RNA transcribed from the vector.